The process of burning sludge is known. In that process, toxic heavy metals are converted from sludge into the gas phase at high temperatures of combustion from 700.degree. to 900.degree. C. (973 K to 1173 K) and are discharged to the atmosphere in an extremely poisonous form as heavy-metal oxides. Furthermore, a great deal of additional energy is required for a thermal afterburning of the fumes. When sludge is burned in fluid-bed furnaces with afterburning chambers, the waste heat of the fumes cannot be used without encountering problems because the fumes are heavily laden with flue dust. Water-injection coolers which are used absorb a considerable portion of the available energy. Purifying large amounts of such fumes, which contain from 20 to 40% of water vapor, is extraordinarily expensive.
First air-drying sludge and then heating it to at least 900.degree. to 1,200.degree. C. (1173 to 1473 K) in an externally heated retort furnace is known from Swiss Pat. No. 478 216. In that process, a considerable portion of the sludge is vaporized. The hot, purified gas is cooled in a heat exchanger to extract steam, and the cooled gas is then available as heating gas.
A procedure and an installation for producing flammable gases from lumpy domestic and industrial refuse are known from U.S. Pat. No. 4,142,867. In that process, the lumpy wastes are first carbonized in rotating drums by indirect heating in the absence of air at temperatures between 300.degree. and 600.degree. C. (573 and 873 K). Next, the carbonization gases are decomposed into combustible gases, at temperatures between 1,000.degree. and 1,200.degree. (1273 and 1473 K), while supplying air which has been preheated by the hot combustible gases that have been generated and low-temperature coke obtained in the first step. The recovery of valuable materials from the lumpy wastes in the second step of the procedure is important in this process. This procedure, which requires rotating drums in the first step, is not suitable for incinerating pure sludge without domestic refuse.
The decomposing of high-polymer, solid waste materials such as polyethylene, polypropylene, etc., into gaseous and liquid fuels with heat and later cooling the so-generated gases is known from U.S. Pat. No. 3,947,256. In that process, the waste materials are melted by heating and compressing in an initial area in a heated extruder, heated to a higher temperature and partially broken up in a second area and vaporized by overheating with simultaneous low pressure in a third area. This procedure is not suitable for incinerating sludge.
Drying sludge in a rotary drier and then using it as fertilizer is known from VDI-Nachrichten No. 43 of Oct. 26, 1979. This procedure consumes a lot of energy.